Rock bolts have been used for many years to provide support to walls and roofs in excavations, mine tunnels and other energy absorptive substrates. The bolts are often required to support very large loads. Failure to provide this support could result in injury to workers. Hence, it is important to know if the rock bolt has been properly installed. Additionally, it is desirable to determine that adequate support is being maintained at some later time after installation.
Rock bolts are generally of two types. A first type of rock bolt is the expansion shell type which uses a long bolt onto the end of which is threaded a fluted, serrated shell. This assembly is inserted into a predrilled hole in the rock and the bolt is tightened, thereby causing the shell to expand against the sides of the hole. In this manner, the tension in the bolt helps to support the rock. Present installation procedures require that the bolt be tightened to a predetermined level of torque, dependent upon such factors as rock consistency, bolt diameter, etc. Because torque is only roughly related to bolt tension (the principal variable being the thread coefficient of friction), it is an inadequate indicator of proper installation.
The second type of rock bolt is the grouted bolt. The grouted bolt includes a long cylindrical piece of metal with convolutions along its periphery similar to a rebar. The bolt is inserted into a predrilled hole in the rock along with an appropriate bonding agent such as a polyester resin. When the bonding agent has cured, the several strata of rock through which the bolt passes will effectively be joined as one with the resultant structure being stronger than any of the individual components. Where testing is employed, a torque test of 150 inch pounds is most commonly used. It has been calculated that a 3/4 inch diameter bolt bonded over only 21/2 inches of its entire length (which is normally 2-8 feet) will satisfy this requirement. Again, torque is an inadequate indicator of proper installation.
Even if proper installation has been assured, rock bolts must be periodically reinspected to ensure that adverse environmental effects such as spalling of the rock, vibration from blasting and heavy equipment, stress-corrosion cracking, etc., have not reduced the load holding capabilities of the bolts.
Heretofore, it has been suggested to evaluate the effectiveness of a rock bolt attachment by acoustic monitoring. A first group of prior art teachings are directed toward monitoring a bolt attachment by detecting a change in length of the bolt based on acoustic parameters such as impedance change (U.S. Pat. No. 3,307,393); pulse transit time (U.S. Pat. No. 3,759,090); frequency change (U.S. Pat. No. 4,062,227). The change of length of the bolt is purportedly indicative of the bolt tension. Another group of prior art teachings in the area of acoustic analysis are directed toward ascertaining a change in resonance due to the coupling of the rock to the bolt via the resin (U.S. Pat. No. 4,062,229).